How to start multiple SSH sessions with single password?

Ubuntu has a package called sshpass. Install it on Ubuntu 16.04 like this:

sudo apt install sshpass

And use it like this:

sshpass -p 'YourPassword' ssh user@host

For multiple hosts you can do something like:

for host in hostname1 hostname2 hostname3; do \
    sshpass -oStrictHostKeyChecking=no \
    -p 'YourPassword' ssh user@${host} "hostname; df -h; echo"; done

The StrictHostKeyChecking disables the usual SSH check that the remote machine is in your list of known hosts; if you are deploying a Pi farm this will probably be the case. When using sshpass, instead of displaying the message 'The authenticity of host xxxx can't be established', the command silently fails without this option.